MEMS actuator with lower power consumption and lower cost simplified fabrication

ABSTRACT

A microactuator device is disclosed that includes a plurality of generally parallel thin flexible sheets bonded together in a predetermined pattern to form an array of unit cells. Preferably, each of the sheets has only a single electrode layer located on one side of the sheet. Pairs of such sheets are then bonded together at spaced bonding locations with the electrode layers facing one another. Several sets of such sheet pairs can then be bonded together to form a microactuator device.

RELATED APPLICATIONS

This Application is related to U.S. patent application Ser. No.09/609,726, now U.S. Pat. No 6,255,758 to Cabuz et al., filed Dec. 29,1999, entitled “POLYMER MICROACTUATOR ARRAY WITH MACROSCOPIC FORCE ANDDISPLACEMENT”; and to co-pending U.S. patent application Ser. No.09/476,667 to Homing, filed Dec. 30, 1999, entitled “MICROACTUATOR ARRAYWITH INTEGRALLY FORMED PACKAGE”, both of which are incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates to microactuators having macroscopic forceand displacement. More particularly, the invention relates tomicroactuators comprising a 3-D array of small actuator cells, which areformed from stacked substrate sheets having an electrode layer on onlyone side of the substrate sheet.

BACKGROUND OF THE INVENTION

Most microactuator arrays, used as MEMS(Micro-Electro-Mechanical-Systems) devices, are fabricated in silicon.Despite the many favorable attributes of silicon, it is not always asuitable or ideal material for every application of MEMS. Silicon isbrittle and subject to breaking, particularly as the total device sizeincreases. This brittleness limits devices, especially actuators, torelatively small sizes capable of only small displacements and forces.The shapes that can be realized in silicon are typically restricted bycrystalline planes or 2-D fabrication processes, and more complicatedstructures often result in prohibitively high cost and low yield. Itwould be of great advantage to the art if another material, other thansilicon, could be used for MEMS and actuators. In particular, it wouldbe advantageous if the structure used to fabricate the actuator requiredsignificantly fewer electrode layers with no substantial loss in force,while requiring less power to operate.

SUMMARY OF THE INVENTION

The present invention provides an electrostatic microactuator arraydevice including a plurality of sheet pairs secured together over thesheet pair lengths and widths. The sheet pairs can be formed of a firstsheet and a second sheet, wherein the first and second sheets have afront and back surface. The first and second sheets preferably have asubstrate layer disposed toward the sheet back surface, a dielectriclayer disposed toward the sheet front surface, and a conductive layerdisposed between the substrate layer and the dielectric layer. Voltageapplied to the conductive layers causes an attractive electrostaticforce between the first and second sheets. The first and second sheetfront surfaces are secured together at a number of spaced bondinglocations along the sheet lengths and widths, such that the first andsecond sheets have a plurality of non-bonded regions between the bondinglocations. In this configuration, the sheet pairs have outer surfacesformed of the back surfaces of the first and second sheets. The outersurfaces of the adjacent sheet pairs are secured together over the sheetpair lengths and widths to form an array of sheet pairs.

In one illustrative embodiment, the sheet pairs are bonded togetheralong continuous bonded locations extending across the sheets. Thesebonded locations may be disposed along substantially straight lines orcurved lines, as desired. Alternatively, the bonded locations may be adiscontinuous series of spots or lines extending over the sheet surface.

The first and second sheets may lie substantially flat between thebonding regions, relying on an applied tension to the microactuator toseparate the sheets. Alternatively, the sheet pairs may be pre-shaped toform cavities between the bonded regions, even with no applied tensionto the microactuator. This latter configuration may be accomplished inany number of ways. For example, the first sheet may be curved, and thesecond sheet may be substantially flat. In this configuration, the firstsheet undulates away from the second flat sheet between the bondedlocations. In another example, both the first and second sheets maycurve away from each other in the non-bonded regions, coming backtogether at the bonded locations. The curved or corrugated sheets maythus form cavities between the bonding regions even with no appliedtension to the microactuator. In any event, it is contemplated that thesheet pairs may collectively be aligned substantially along a plane, ormay conform to a curved surface.

The sheets included in the present invention require only a singleelectrode layer bonded or applied to a substrate layer. The electrodelayer is preferably formed of a conductor material deposited over thesubstrate layer, and a dielectric material deposited over the conductor.The substrate layer may be a polymer, ceramic, silicon, steel, or anyother material that provides the desired characteristics for aparticular application. The first and second sheets of each pair can bebonded together so that the front surfaces of the sheets are in closeproximity to one another near the bonded regions. The close proximity ofthe sheets helps overcome the initial separation of the non-bondedregions of the sheets, and zip together inward from the bondinglocations toward the center of the non-bonded regions. The attraction ofthe sheets for each other upon the application of an electricalpotential provides the force for the microactuator.

In one method of manufacture, the sheet pairs are formed by firstbonding together the sheets at periodic intervals, for example, alongregularly spaced parallel lines. This can be done using manufacturingtechniques such as heat bonding or adhesive application. With the sheetpairs formed, the sheet pairs can be bonded together many at a time, toform the microactuator array. The process of bonding together themultiple, already formed sheet pairs can be less precise with little orno loss in device performance than the process used to bond the sheetsof a sheet pair. Since no voltage is applied between the sheet pairs,the distance between the sheet pairs may not be as critical as thedistance between the sheets forming each pair.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cut-away perspective view of a microactuator usingpolymer sheets;

FIG. 2 is a schematic, partially cut-away and in section view of amicroactuator formed of a plurality of unit cells with a center polymersheet;

FIG. 3 is a schematic, partially cut-away and in section view of amicroactuator formed of a plurality of unit cells with no center polymersheet;

FIG. 4 is a fragmentary, schematic, cross-sectional view of unit cellsof FIG. 2 formed of two polymer sheet pairs having only a single surfaceelectrode layer on each sheet with one sheet pre-curved or allowed tobend and the other remaining substantially flat;

FIG. 5 is a fragmentary, schematic, cross-sectional view of unit cellsof FIG. 3 formed of two polymer sheet pairs having only a single surfaceelectrode layer with both sheets either pre-curved or allowed to bend;

FIG. 6 is a fragmentary, schematic, cross-sectional view of part of theunit cell of FIG. 4;

FIG. 7 is a fragmentary, schematic, cross-sectional view of part of theunit cell of FIG. 5;

FIG. 8 is a fragmentary, schematic, cross-sectional view of part of theunit cell of FIG. 5 with a trenched bond;

FIG. 9 is a diagram showing the model parameters for single-sidedelectrodes; and

FIG. 10 is a diagram showing the model parameters for double-sidedelectrodes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a partial cut-away perspective view of a microactuator usingpolymer sheets. The illustrative microactuator is formed by stacking andbonding together multiple polymer sheets, as described in for example,co-pending U.S. patent applications Ser. No. 09/609,726, now U.S. Pat.No. 6,255,758 to Cabuz et al., filed Dec. 29, 1999, entitled “POLYMERMICROACTUATOR ARRAY WITH MACROSCOPIC FORCE AND DISPLACEMENT”, and U.S.patent application Ser. No. 09/476,667 to Homing, filed Dec. 30, 1999,entitled “MICROACTUATOR ARRAY WITH INTEGRALLY FORMED PACKAGE”, both ofwhich are incorporated herein by reference.

As described in the above references, electrodes from selected sheetscan be electrically tied together or can be individually addressed,depending on the degree of control and sophistication of the end use,noting that individual addressing requires more connections, so it wouldinvolve higher cost but finer control when needed. Electrical contactbetween layers and sheets can be made through flexible plasticinterconnect straps, as described in U.S. patent applications Ser. No.09/609,726 now U.S. Pat. No. 6,255,758 to Cabuz et al., or through anintegrally formed package, as described in U.S. patent application Ser.No. 09/476,667 to Homing.

In either case, the top of the microactuator stack may be secured to anupper housing 42, and the bottom of the microactuator stack is securedto a lower housing 48. The upper and/or lower housings 42 and 48 mayinclude an external connector 50 and control circuitry 52 forcontrolling the microactuator. The upper and/or lower housing 42 and 48may further include one or more levels of metal interconnects, as in aprinted circuit board, to route the inputs of the external connector 50through the control circuitry 52 and to the sheets of the actuator usingone or more of the flexible interconnect straps.

Referring now to FIG. 2, a microactuator 70 is illustrated extendingfrom an upper housing 72 to a lower housing 74. The terms “upper” and“lower” are used herein for purposes of illustration only, as themicroactuators in the general case may be used in any position. Upperhousing 72 includes an upper connector 73 and lower housing 74 includesa lower connector 75, which can both be used for connecting themicroactuator to other members to which force is to be applied. In oneembodiment, upper housing 72 has an upper connect layer 76 and lowerhousing 74 has a lower connect layer 78, both of which can be used toelectrically connect the electrostatically moveable unit cells andpolymeric sheets discussed further below.

The embodiment illustrated in FIG. 2 includes alternating layers ofpolymeric sheets which can be viewed as forming repeating unit cells 86that form the moveable elements of the microactuator. Microactuator 70includes a first curved or bent upper polymer sheet 80, a substantiallyflat lower polymeric sheet 82, and a second curved or bent upperpolymeric sheet 84, etc. In this embodiment, the curved sheets may besimilar or identical, and each curved sheet may be more properly viewedas grouped together with the flat sheet beneath each curved sheet toform a sheet pair. The pairs may then be viewed as being securedtogether to form the microactuator. The polymeric sheets repeat innumerous layers, with only the upper most layers illustrated in FIG. 2,the remainder being indicated only by repetition marks 88. In apreferred embodiment, the upper and lower sheets have a substantialdepth, giving rise to a three-dimensional structure.

First upper sheets 80, first lower flat sheets 82, and second uppersheets 84 are electrically connected to the upper housing 72 throughelectrical connection lines 90. Electrical connection lines 90 are shownin a highly diagrammatic form, and can be used to interconnect the upperand lower sheets in various combinations, as illustrated by straps 40and 44 in FIG. 1. A protective film or envelope 91 encloses themicroactuator in some embodiments, protecting the polymeric sheets fromthe environment. In use, microactuator 70 operates by the attraction ofupper polymeric sheet 80 to lower sheet 82, and by the attraction ofsecond curved upper polymeric sheet 84 to the flat polymeric sheetbeneath it. The electrostatic attractive forces between the sheets pullthe sheets closer together, acting to pull upper housing 72 toward lowerhousing 74.

Referring now to FIG. 3, another embodiment of the invention isillustrated in a microactuator 92 also having upper housing 72, lowerhousing 74, electrical interconnect lines 90, and protective outer film91. Microactuator 92, unlike microactuator 70, has no flat polymericsheet. Microactuator 92 includes a series of curved or bent upperpolymeric sheets 96 and curved or bent lower polymeric sheets 98.Microactuator 92 may be viewed as being formed of numerous repeatingunit cells 94 bonded together. In particular, the polymeric sheets maybe viewed as being secured together in pairs, with the assembled pairsthen secured together to form the microactuator. In the embodimentillustrated, the upper and lower polymeric sheets 96 and 98 are directlysecured to each other using a bonding process, which can includeadhesives or other methods for securing polymers to polymers, as furtherdescribed below. In use, microactuator 92 operates by the attraction ofupper polymeric sheet 96 to lower polymeric sheet 98. The electrostaticattractive forces between the sheets pull the sheets closer together,acting to pull upper housing 72 toward lower housing 74.

Referring now to FIG. 4, a unit cell such as unit cell 86 of FIG. 2 isillustrated in greater detail. First curved upper sheet 80 and firstflat lower sheet 82 are also formed of an electrode layer such aselectrode layer 105 adhered to polymeric layer 104, with electrode layer105 having conductive layer 103 and dielectric layer 102. A second lowerflat sheet 89 is shown beneath the second upper curved sheet 84. Firstupper curved sheet 80 and first lower flat sheet 82 may be considered asa sheet pair 83. Likewise, second upper curved sheet 84 and second flatlower sheet 89 may be considered another sheet pair 85. Thus, themicroactuator portion illustrated in FIG. 4 may be considered to beformed of two sheet pairs 83 and 85. The opposing electrode layers ofeach opposing sheet in the pair may be seen to form cavities 87 whichcontract, thereby causing displacement of the actuator.

Referring now to FIG. 5, a unit cell such as unit cell 94 of FIG. 3 isillustrated in greater detail. Upper polymeric sheets 96 and lowersheets 98 include a polymeric layer 112 adjacent to and coupled to anelectrode layer 115 which, in this embodiment, is formed of a conductivelayer 111 and a dielectric layer 114. Each upper sheet 96 and lowersheet 98 may be viewed as being secured together to form a sheet pair,such as sheet pairs 93 and 95 illustrated in FIG. 5. Numerous sheetpairs may be secured together to form the microactuator. Cavities 97 areformed between the curves or corrugations of the upper and lower sheetsof each sheet pair. Cavities 97 contract with the application ofelectrical potential, causing displacement of the actuator.

Referring now to FIG. 6, part of a unit cell having a flat polymericsheet such as unit cell 86 of FIGS. 2 and 4 is illustrated. The unitcell includes first flat lower sheet 82 at top and second lower flatsheet 89 at bottom, with the second curved upper sheet 84 extendingtherebetween. In the illustrative embodiment, second curved upper sheet84 is adhered to first upper flat sheet 82 with upper adhesive 116, andis further adhered to the second upper flat sheet 89 with lower adhesive118. Other methods for securing the second curved upper sheet 84 tofirst upper flat sheet 82 and/or to the second upper flat sheet 89 mayinclude thermal sealing, ultrasonic welding, etc., particularly when thesheets are formed from materials that can be directly bonded together.

Second upper curved sheet 84 and second lower flat sheet 89 may beconsidered as forming a sheet pair having cavity 87 therebetween. Thecontraction of cavity 87 may be seen to provide the actuatordisplacement. In this view, first lower flat sheet 82 may be seen toform the lower part of another sheet pair. Numerous lower adhesives 118may be viewed as periodically bonding the two sheets of the sheet pairtogether, forming a non-bonded region 87 therebetween. Upper adhesive116 may be viewed as bonding two sheet pairs together.

Electric field lines 114 are illustrated in the region where conductivelayers 103 of the curved upper sheet 84 and the flat sheet 89 come closetogether, creating as small of a gap as possible between the twodielectric layers 102. As can be seen from inspection of FIG. 6, thethickness of lower adhesive 118 can establish the limit of the gapthickness between the two sheets. The tolerances and limit of the smallgap between the sheets may be important in manufacturing themicroactuators. The electrostatic attraction between the two closelyspaced electrodes is dependent on the separation distance. As curvedupper sheet 84 is pulled down toward flat sheet 89, the gap between thetwo electrodes is decreased in the region more toward the center of theunit cell, which increases the attraction in the region more toward thecenter of the unit cell. The process of closing the gap from the outsidein acts as a rolling or zipping action, acting to pull the previouslyspaced apart sheets together.

The individual sheets may be preformed using low cost processes, such ascutting or stamping rather than photolithography and etching. Aluminumor other metal or alloy electrodes and one or more dielectric films,such as aluminum oxide, polyimide, polyvinylidene flouride (PVdF),acrylates, or other suitable organic or inorganic dielectrics, may beapplied to the sheets. The conductive portion forms the electrode, andthe dielectric prevents shorting of the electrodes when they touch.These films can be patterned using standard patterning techniques, suchas those used to make printed circuit boards or roll type printingprocesses. The dielectric layer should produce a chemically stablesurface with a very low surface energy. This may be helpful inpreventing stiction. The level of performance that can be obtained froman electrostatic actuator often depends in part on the dielectricstrength of the material used as a dielectric.

Referring now to FIG. 7, part of a unit cell not having a centerpolymeric sheet, such as unit cell 94 of FIGS. 3 and 5, is illustrated.The unit cell includes curved upper sheet 96 and curved lower sheet 98.Upper sheet 96 is shown adhered to the lower sheet 98 with adhesive 120.Upper sheet 96 and lower sheet 98 may be viewed as forming a sheet pair,where such sheet pairs are secured together to form a microactuator.Adhesive 120 can be viewed as periodically bonding the sheet pair 96 and98 together, forming a non-bonded region 97 therebetween. Electric fieldlines 114 are illustrated in the region where conductive layers 111 ofupper polymeric sheet 96 and lower polymeric sheet 98 come closelytogether, creating a small gap between the two conductive layers.

As can be seen from inspection of FIG. 7, the thickness of adhesive 120can cause a gap between the two sheets. As discussed with respect toFIG. 6, the tolerances and limit of such a gap may be important inmanufacturing the microactuators, as the electrostatic attractionbetween the two closely spaced electrodes is dependent on the separationdistance. To minimize the gap between the two sheets, a trench may becut or etched into one or both of the sheets at the bonding locations.FIG. 8 shows a bond where a first trench 130 is cut into the upper sheet96 and a second trench 132 is cut into the lower sheet 98. The firsttrench 130 and the second trench 132 are preferably in registration, asshown, and extend into the substrate of the sheet. In one embodiment,the first trench 130 may extend all the way through the upper sheet 96,as shown by dotted line 133. Likewise, the second trench 132 may extendall the way through the lower sheet 98, as shown by dotted line 135.

An adhesive 134 may be provided in the cavity formed by the first trench130 and/or the second trench 132. Once the adhesive is provided, the twosheets 96 and 98 may be pressed together to form the bond. The firsttrench 130 and the second trench 132 may have a length. The adhesivepreferably does not extend the full length of the trenches. This mayhelp prevent excess adhesive from flowing outside of the trenches andbetween the upper and lower sheets. In some embodiments, a catalyst maybe provided to help cure the adhesive. That catalyst may be, forexample, heat, UV radiation, pressure, etc. An advantage of thisapproach is that the trenches 130 and 132 provide a cavity for theadhesive 134, thereby leaving little or no gap between the sheets. Thismay increase the performance of the microactuator, as further describedherein. Another advantage of this approach is that the concentration ofstress at the bond may be reduced.

The electrostatic field (and therefore the electrostatic energy) dropsoff extremely rapidly as the separation between the sheets increases.Simple estimates and finite element models show that the electrostaticfield is negligible when the gap is a little over 1 μm, using typicalvalues of 0.3 μm for the dielectric thickness and 3.0 for the dielectricconstant. Therefore, the electrostatic energy can be simplified, withlittle loss in accuracy, by assuming it is zero everywhere that the gapis nonzero, and is constant where the gap is zero. The electrostaticenergy at a fixed voltage is then simply that of a parallel platecapacitor, and can be expressed using the relation: $\begin{matrix}{U_{E} = \left\{ \begin{matrix}{- \quad \frac{ɛ_{o}ɛ\quad {b\left( {l - c} \right)}V^{2}}{4d}} & {{for}\quad {single}\quad {sided}\quad ({ss})\quad {electrodes}} \\{- \quad \frac{\quad {ɛ_{o}ɛ\quad {b\left( {2a} \right)}V^{2}}}{4d}} & {{for}\quad {double}\quad {sided}\quad ({ds})\quad {electrodes}}\end{matrix} \right.} & \left( {{Equation}\quad 1} \right)\end{matrix}$

The term “single sided electrode” refers to a unit cell that has anelectrode layer on only one side of the sheets. The term “double sidedelectrode” refers to a unit cell that has an electrode layer on bothsides of the sheets.

FIG. 9 is a diagram showing the model parameters used for unit cellsmade from sheets with a single-sided electrode. In the design withoutflat center planes (e.g. FIG. 7), the displacement δ is still defined asthe distance between the two surfaces when the structure is extended.FIG. 10 is a diagram showing the model parameters used for unit cellsmade from sheets with double-sided electrodes.

The bending energy is derived from the curvature of the sheet. Since theelectrostatic force falls off so quickly, it has very little influenceon the bending profile of the sheets, although it has a strong influenceon the position of the point of contact between the two sheets and,therefore, the length of the bent region. Thus, the profile is that of abeam with a load W at the end. The bending energy can thus be expressedusing the relation: $\begin{matrix}\begin{matrix}{U_{B} = {\frac{{Ebt}^{3}}{24}\quad {\int{\left( \frac{\partial^{2}y}{\partial x^{2}} \right)^{2}{x}}}}} \\{= \left\{ \begin{matrix}{\frac{1}{2}\quad {{kb}\left( \frac{l}{c} \right)}^{3}\delta^{2}} & {{for}\quad {ss}\quad {electrodes}} \\{\frac{1}{2}\quad {{kb}\left( \frac{l}{l - {2a}} \right)}^{3}\delta^{2}} & {{for}\quad {ds}\quad {electrodes}}\end{matrix} \right.}\end{matrix} & \left( {{Equation}\quad 2} \right)\end{matrix}$

where E is Young's modulus and k is the “spring constant” of thestructure. $\begin{matrix}{{k \equiv \frac{W}{b\quad \delta}} = \left\{ \begin{matrix}\frac{{Et}^{3}}{l^{3}} & {{with}\quad {flat}\quad {centerplanes}} \\\frac{{Et}^{3}}{2l^{3}} & {{without}\quad {flat}\quad {centerplanes}}\end{matrix} \right.} & \left( {{Equation}\quad 3} \right)\end{matrix}$

The total energy can be expressed using the relation: $\begin{matrix}\begin{matrix}{U_{Total} = {U_{B} + U_{E}}} \\{= \left\{ \begin{matrix}{{\frac{1}{2}\quad {{kb}\left( \frac{l}{c} \right)}^{3}\delta^{2}} - \frac{ɛ_{o}ɛ\quad {b\left( {l - c} \right)}V^{2}}{4d}} & {{for}\quad {ss}\quad {electrodes}} \\{{\frac{1}{2}\quad {{kb}\left( \frac{l}{l - {2a}} \right)}^{3}\delta^{2}} - \frac{ɛ_{o}ɛ\quad {b\left( {2a} \right)}V^{2}}{4d}} & {{for}\quad {ds}\quad {electrodes}}\end{matrix} \right.}\end{matrix} & \left( {{Equation}\quad 4} \right)\end{matrix}$

Equilibrium is achieved when ∂U_(Total)/∂C=0 for single sided electrodesor ∂U_(Total)/∂a=0 for double sided electrodes, where the partialderivatives are taken while holding the displacement, δ, constant. Thisyields the same answer for both single sided (ss) and double sided (ds)cases: $\begin{matrix}{\frac{W}{b} = \left\{ \begin{matrix}{\left( \frac{l}{6} \right)^{\frac{3}{4}}\quad \frac{k^{\frac{1}{4}}}{\delta^{\frac{1}{2}}}\left( \frac{ɛ_{o}ɛ\quad V^{2}}{d} \right)^{\frac{3}{4}}} & {V > V_{PI}} \\{k\quad \delta} & {V < V_{PI}}\end{matrix} \right.} & \left( {{Equation}\quad 5} \right) \\{and} & \quad \\{V_{PI} = {\left( \frac{6d}{{kl}\quad ɛ_{o}ɛ} \right)^{\frac{1}{2}}\frac{W}{b}}} & \left( {{Equation}\quad 6} \right)\end{matrix}$

The pull-in voltage, V_(PI), is the applied voltage at which theactuator rolls or zips closed, which corresponds to the voltage at whichthe two expressions for W/b in Equation (5) are equal. These expressionsare valid for both models—with and without centerplanes.

At 0 V, the displacement is defined solely by the spring constant of thestructure. If a fixed external load W is applied to the actuator, itwill open to a displacement δ. As the voltage is increased from zero, anelectrostatic force develops, this electrostatic force is not largeenough to move the actuator. However, once the pull-in voltage V_(PI) isreached, the displacement δ begins to decrease. This increases theactuator force even more, while the external load remains fixed at W, sothe actuator pulls completely in.

Since the expression for force (Equation 5) is identical for single anddouble sided designs, it can be concluded that only single sidedelectrodes are needed. This has a number of advantages. First, singleside electrode processing is easier and cheaper than double sideelectrode processing. Double side electrode processing typically exposesthe electrode and dielectric on one side to scratching, etc. whileprocessing the other side. This may reduce the yield relative to asingle side electrode process. In addition, many polymer sheets have oneside that is rougher than the other. Single sided electrode processingallows only the smooth side to be processed. This is particularlyimportant since the spacing between layers can be important to theoperation of the actuator.

Another advantage of providing a single sided electrode is that theinput power requirements may be reduced. The energy expended by theelectrical power source in actuating a load is the sum of the mechanicalwork done on the load and the electrical energy stored in the capacitor:U=Wδ+CV²/2. The capacitance of the actuator with double-sided electrodesis twice that of the actuator with single-sided electrodes. Therefore,twice as much electrical energy must be stored in the actuator withdouble-sided electrodes. Yet no additional mechanical work is performed.Further, fewer electrical interconnects may be needed.

Yet another advantage of using single-sided electrodes is that themanufacturing process may be more readily controllable with regard tothe thickness of the adhesive near the gap between facing electrodes ofeach sheet pair. In particular, in FIG. 6, the thickness of loweradhesive 118 is much more important than the thickness of upper adhesive116, which does not control the gap between facing electrodes. Upperadhesive 116 bonds adjacent pairs of contracting sheets. Similarly, inFIG. 7, the thickness of adhesive 120 may be critical in establishingthe gap between facing electrodes, but the adhesive securing thenon-electrode faces of the adjacent sheets bonds together adjacent pairsof contracting sheets. In manufacture, the two facing electrode layerscan be carefully laminated using a first process designed to control thethickness of the adhesive. In one method, adhesive is not used at all,and the two facing electrode sheets are joined together using analternate method such as heat bonding requiring little or no adhesive,insuring a very close gap between the electrode layers. The resulting“sheet pairs” can then be secured together, back to back, using a secondmethod designed to preserve the previously formed bonds between sheets,at the possible expense of control of adhesive thickness or absoluteadhesive thickness.

The force equation (5) above can also be used to compare the designswith and without the flat centerplane. From the spring constantequations (Equation 3), an actuator without centerplanes has a springconstant half that of an actuator with centerplanes. That is, they havedifferent mechanical responses. If the actuator without centerplanes ismodified so that its length is l/(2)^(⅓), the two actuators have thesame spring constant. Thus, at zero voltage, the two structures haveexactly the same response (restoring force and displacement) to anexternally applied load. For a given applied force, the pull-in voltage(Equation 6) of the design without the centerplanes is only (2)^(⅙)=1.12times the pull-in voltage of the design with centerplanes, since V_(PI)has a dependence on length. However, the small increase in pull-involtage is outweighed by the much simpler fabrication of the designwithout centerplanes.

Numerous advantages of the invention covered by this document have beenset forth in the foregoing description. It will be understood, however,that this disclosure is, in many respects, only illustrative. Changesmay be made in details, particularly in matters of shape, size, andarrangement of parts without exceeding the scope of the invention. Theinvention's scope is, of course, defined in the language in which theappended claims are expressed.

What is claimed is:
 1. An actuator array device, comprising: a pluralityof sheets bonded together in a predetermined pattern to form an array ofunit cells; and selected sheets having a substrate with two opposingsurfaces, a first one of the opposing surfaces having an electrode layeradjacent thereto and the other opposing surface being generally freefrom an electrode layer, wherein the electrode layer is one of aplurality of electrodes associated with said array of unit cells,selected sheets being bonded together with the first opposing surfacesfacing one another.
 2. An actuator as recited in claim 1, wherein all ofsaid sheets have only a single electrode layer.
 3. An actuator asrecited in claim 1, wherein said electrode layer of the selected sheetsis disposed near the surface of each sheet.
 4. An actuator as recited inclaim 1, wherein said electrode layer is a thin electrically conductivelayer deposited on said sheet and a dielectric layer thereover.
 5. Anactuator as recited in claim 1 wherein the sheets are thin polymersheets.
 6. An actuator as recited in claim 1 wherein the sheets areceramic sheets.
 7. An actuator as recited in claim 1 wherein the sheetsare silicon sheets.
 8. An actuator as recited in claim 1 wherein thesheets lie generally along a plane.
 9. An actuator as recited in claim 1wherein the sheets lie generally along a curved surface.
 10. Anactuator, comprising: a plurality of sheet pairs having a sheet pairlength; said sheet pairs including a first sheet and a second sheet eachhaving a sheet length, wherein said first and second sheets have a frontsurface and a back surface; said first and second sheets have asubstrate layer disposed toward said back surface, a dielectric layerdisposed toward said front surface, and a conductive layer disposedbetween said substrate layers and said dielectric layers; and said firstand second sheet front surfaces being secured together at spaced bondingregions along said sheet length, such that said first and second sheetshave non-bonded regions formed between said plurality of spaced bondedregions.
 11. An actuator as recited in claim 10, wherein saidnon-bonding regions form a plurality of cavities between said first andsecond sheets with no external force applied to the actuator.
 12. Anactuator as recited in claim 11, wherein said first sheet is shaped tocurve away from said second sheet in said non-bonding regions with noexternal force applied to the actuator.
 13. An actuator as recited inclaim 12, wherein said second sheet is substantially flat and lies alonga plane defined by said bonding regions.
 14. An actuator as recited inclaim 11, wherein, in said non-bonding regions, said first and secondsheets are shaped to curve away from a plane defined by said bondingregions.
 15. An actuator as recited in claim 10, wherein said first andsecond sheets are both substantially flat and parallel when no externalforce is applied to the actuator.
 16. An actuator as recited in claim 15wherein said first and second sheets become separated in the non-bondingregions when an external force is applied to the actuator.
 17. Anactuator as recited in claim 10 wherein the first and second sheetscollectively lie generally along a plane.
 18. An actuator as recited inclaim 10 wherein the first and second sheets collectively lie generallyalong a curved surface.
 19. An actuator array device, comprising: aplurality of first sheets having sheet lengths; a plurality of secondsheets having sheet lengths; wherein each of said first and secondsheets having a front side and a back side with an electrode layerpositioned adjacent the front side but generally free from an electrodelayer on the back side; pairs of said first and second sheets areoperably secured together at spaced bonding regions with their frontsides facing one another to form a plurality of sheet pairs; andadjacent sheet pairs are operably secured together.
 20. An actuatorarray device as recited in claim 19, wherein said second sheets aresubstantially flat.
 21. An actuator array device as recited in claim 20,wherein said first sheets have a plurality of repeating bends, such thatsaid first sheets have peaks and troughs relative to said second sheets.22. An actuator array device as recited in claim 20, wherein both saidfirst and second sheets are substantially flat.
 23. An actuator arraydevice as recited in claim 19, wherein both said first and second sheetshave a plurality of repeating bends.
 24. An actuator array device asrecited in claim 19, wherein the first and second sheets of each sheetpair have repeating cycles of small to large distances therebetween,said large distances being made small by the operation of electrostaticattractive force between said first and second sheets.
 25. An actuatorarray device as recited in claim 19, wherein each of said electrodelayers includes a conductive layer having a dielectric layer thereover.26. An actuator array device as recited in claim 25, wherein saidconductive layer is a metallic layer applied to said sheet.
 27. Anactuator, comprising: a plurality of sheet pairs having a sheet pairlength; said sheet pairs including a first sheet and a second sheet eachhaving a sheet length, wherein said first and second sheets have a frontsurface and a back surface; said first and second sheets have asubstrate layer disposed toward said back surface, a dielectric layerdisposed toward said front surface, and a conductive layer disposedbetween said substrate layers and said dielectric layers; said first andsecond sheet front surfaces having one or more trenches defined thereinat spaced bonding regions, and an adhesive disposed in selectedtrenches; and said first and second sheet front surfaces being securedtogether at the spaced bonding regions by the adhesive in the trenches,such that said first and second sheets have non-bonded regions formedbetween said plurality of spaced bonded regions.
 28. An actuatoraccording to claim 27 wherein said sheet pair back surfaces are securedto adjacent sheet pair back surfaces at spaced pair-to-pair bondingregions along the sheet pair lengths.